One characteristic parameter of cancer cells is their higher than normal fraction of cells in S-phase. Thus, various attempts have been made to develop “S-phase specific” anti-cancer drugs. For example, Camptothecin, which is a topoisomerase I inhibitor, showed S-phase specific cytotoxicity.
Another equally essential biochemical event for DNA replication during S-phase is homologous recombination (HR), which is used to restart the collapsed or stalled replication fork. When double-stranded DNA breaks occur at G1 or M phase of mammalian cells, non-homologous end joining (NHEJ) plays a major role in repairing those breaks. One possibility to explain this discrepancy lies in the presence of required factors for HR in S phase. Among a number of genes critical for HR in eukaryotes, Rad51 recombinase plays a quintessential role. Rad51 multimerizes to form a nucleoprotein filament on single-stranded DNA and catalyzes homologous DNA pairing and strand exchange in vitro. After treatment with DNA damaging agents, Rad51 foci can be observed at the sites of DNA lesion along with other HR-related proteins such as BRCA1, BRCA2, BLM and RPA. In many cancer cells, enhanced HR and increased expression of Rad51 have been observed, while in normal cells, Rad51 expression is restricted to S phase and the recombination frequency and its expression level are much lower than those in cancer cells. Moreover, Rad51 is positively correlated with resistance of cancer cells to DNA damage inducing radio- or chemotherapies, which suggests that elevated levels of Rad51 in the HR pathway are critical for the proliferation of cancer cells rather than normal somatic cells.
Rad51 is also a key effector downstream of BCR-ABL, essential for cell proliferation and survival. Bcr/Abl, a constitutively active fusion tyrosine kinase derived from Philadelphia chromosome (t9:22), is found in most (95%) chronic myelogenous leukemia (CML) and in many acute lymphocytic leukemia (ALL) patient. Bcr/Abl-expressing cells are typically resistant to genotoxic treatments, and it has been shown that Bcr/Abl-Stat5-Rad51 pathway in leukemia cells stimulates homologous recombination (HR), contributing to the resistance to DNA damaging agents. In BCR-ABL positive leukemia cells, Rad51, itself a direct substrate of BCR-ABL, is transcriptionally activated via BCR-ABL-Stat5 pathway. Because of the constitutive activation of BCR-ABL, Rad51 protein level is constantly elevated in these cells, contributing to resistance to chemotherapies and the continuous cell proliferation.
The elevation of Rad51 levels in CML is a major downstream event of Bcr-Abl regulation and contributes to the proliferation and drug resistance of CML cells. Current therapeutics, such as Imatinib treatment, have proven to be successful in the inhibition of BCR-ABL activity, but suffer from relapse after developing new resistance to the drug. Unfortunately, resistance to imatinib occurs frequently (over 70%) in accelerated phase (AP) and myeloid blast crisis (BC) patients, resulting in remissions lasting for only 6-12 months. The major mechanism of acquired resistance to imatinib is mutations in BCR-ABL kinase domain. One of the most serious mutations is T315I, which is responsible for about 15% of imatinib-resistant CML cases. The T315I mutation also confers resistance to dasatinib (BMS-354825) and nilotinib (AMN107), two second generation BCR-ABL inhibitors, which were developed to overcome imatinib-resistance. In addition, resistance to imatinib can also be caused by mutations located outside the kinase domain, which are most likely beyond the scope of more specific BCR-ABL kinase inhibitors. For that reason, developing new approaches to circumvent or counteract resistance to BCR-ABL kinase inhibitors still poses a challenging problem, especially in late phase CML.
Therefore, while there are various drugs and conceptual drug therapies are known in the art, successful implementation is often prevented by development of resistance, or by lack of functional and/or well-tolerated drug candidates. Thus, there is still a need to provide new therapeutic agents to treat CML, and especially treatment resistant CML.